1. Field of the Invention
The present invention relates to an automatic focusing apparatus for a camera in which a flash light is used as an auxiliary light source, for example, when the brightness or contrast of an object is low.
2. Description of the Related Art
In a conventional passive AF (autofocusing) camera, for example, if the brightness or contrast of an object to be taken is low, an auxiliary light source incorporated in the camera body projects a contrast pattern as auxiliary light onto the object, so that a contrast is produced on an object image received by an AF sensor to detect the focused state based on the object image. However, the provision of the focusing auxiliary light source in addition to a photographing flash lamp increases the manufacturing cost. To solve this, it is known to intermittently emit flashes of light by the flash lamp to thereby obtain auxiliary light for the automatic focusing operation, as disclosed in the U.S. Pat. No. 5,448,331.
However, in the emission control system in which flashes of light are intermittently emitted, the integral value of the AF sensor exceeds a processible range by one emission if the object distance is small or the object""s reflectance is high, so that no focusing operation can be carried out. Moreover, if the object distance is large or the object""s reflectance is low, the number of the emissions of auxiliary light is increased, thus leading to an increase in the time necessary to complete an integration process of the AF sensor. Consequently, the significant purpose the auxiliary light, i.e., a quick focus detection and a fast automatic focusing cannot be achieved.
Furthermore, the charge voltage of a voltage doubler condenser to apply a doubled voltage to a xenon tube of the flash lamp is reduced due to the emission of the flash light and is thereafter increased by charging the condenser for a subsequent emission of the flash light. However, if the number of emissions is increased, the power consumption of the flash device is increased, and hence, the time for the subsequent emission of the flash light comes before the charge voltage of the voltage doubler condenser is recovered to a voltage large enough to emit the flash light, thus resulting in a failure of emission. To prevent this, the charge voltage of the flash device at the commencement of the intermittent flashes of light must be set high.
It is an object of the present invention to eliminate the drawbacks of the prior art mentioned above, by providing an automatic focusing apparatus in which the quantity of auxiliary light to be emitted can be precisely controlled, regardless of the conditions of the object to be taken.
To achieve the above mentioned object, an automatic focusing apparatus is provided which includes a focus detector for receiving object light and detecting a focused state of an object, a light emitter for emitting a flash of light toward to the object, and an emission controller for causing the light emitter to intermittently emit flashes of light as an auxiliary light source for detecting the focused state when focus detection cannot be carried out by the focus detector. The emission controller varies stepwise at least one of an emission time and an emission time interval at every predetermined number of emissions, when the light emitter intermittently emits flashes of light as the auxiliary light source.
In an embodiment, the emission controller varies stepwise both the emission time and the emission time interval at every predetermined number of emissions when the light emitter intermittently emits flashes of light as the auxiliary light source.
In an embodiment, the emission controller increases the emission time and/or the emission time interval, as the number of emissions increases.
In an embodiment, the emission time and/or the emission time interval at the commencement of the emission is the controllable shortest time.
Preferably, the emission controller causes the light emitter to intermittently emit flashes of light when the focus detector cannot detect the focused state without emitting auxiliary light.
Preferably, the automatic focusing apparatus is applied to a camera having a controller which moves a focusing lens of a photographing lens in accordance with the detection result of the focus detector, wherein when the focus detector cannot detect the focused state while intermittently emitting flashes of light by the light emitter, the controller moves the focusing lens to a predetermined position and then controls the emission controller so that the focus detector detects the focused state while the emission controller causes the light emitter to intermittently emit flashes of light.
Preferably, the emission controller sets a preset maximum number of the emissions.
Preferably, the focus detector includes an image pickup device which receives object light, converts the object light information into electric signals, and integrates the electric signals as electric charges; and wherein upon intermittent emission of the light emitter, the emission controller stops the intermittent emission of the light emitter when the integral value of the image pickup device reaches a predetermined value before the number of the emissions reaches the preset maximum number.
In an embodiment, the emission time and the emission time interval is defined by a predetermined calculation based on a predetermined reference emission time, a predetermined reference emission time interval, and the number of emissions until the previous emission of the light emitter.
Preferably, the emission time and the emission time interval is defined by the following expressions (1) and (2):
Emission time=reference emission time+correction timexc3x97the emission number÷correction number;xe2x80x83xe2x80x83(1)
and
Emission time interval=reference emission interval+correction intervalxc3x97the emission number÷correction number;xe2x80x83xe2x80x83(2)
wherein the correction time, the correction interval and the correction number are predetermined values.
With the above described arrangements, the quantity of auxiliary light to be emitted by the light emitter can be precisely controlled, regardless of photographic conditions, etc., of the object to be taken.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 11-151589 (filed on May 31, 1999) which is expressly incorporated herein by reference in its entirety.